The present invention relates to a video-emphasis encoding apparatus and a method of video-emphasis encoding for applying an emphasis processing to input video signals and encoding the emphasized signals to obtain video bitstreams. Moreover, this invention relates to a video-emphasis decoding apparatus and a method of video-emphasis decoding for decoding the video bitstreams.
[Emphasis and Encoding]
An emphasis processing is a known technique widely used in storage and transfer of video and audio signals. In detail, an emphasis circuit gives emphasis effects on high-frequency components of video or audio signals before storage and transfer.
The emphasized high-frequency components are eliminated by a deemphasis circuit after storage and transfer for regaining the frequency characteristics which the video or audio signals have exhibited before transferred. Noises and aliasing generated during storage or transfer can be attenuated only by a deemphasis processing.
The emphasis processing could cause an overrange condition on high-frequency component-emphasized video or audio signals against signal storage and transfer.
The emphasis processing is, however, useful in highly-efficient encoding processing such as discrete cosine transform (DCT), for less block distortion and mosquito noises which may otherwise be generated during encoding and decoding. This processing may not achieve high coding efficiency due to large quantization errors which will be produced at a fixed amount of codes to be generated for emphasized high-frequency components that generate a large amount of data. Nevertheless, the emphasis processing eliminates block noises because this processing is applied over blocks, different from DCT-encoding and decoding.
Adaptive filtering for eliminating noises and distortion on decoded pictures (post processing) may not be efficient because signal components could also be filtered during noises/aliasing elimination.
In MPEG-Audio standards for audio encoding, a bitstream is multiplexed with a flag that indicates whether an encoded signal has been applied the emphasis processing. The encoded signal is then applied the deemphasis processing after decoded when it carries a flag indicating the emphasis.
[Enhancement]
An enhancement processing is similar to the emphasis processing but for different purpose, only applied to video signals in general. In detail, the enhancement processing compensates reproduced pictures for detail sections which will be hard to watch from a remote place due to the visual spatial frequency characteristics of human.
The enhancement processing is similar to the emphasis processing because it enhances high-frequency components of video signals. This processing is usually performed at TV cameras and TV sets. The enhancement processing requires no reverse correction, thus having no processing like the deemphasis processing.
[Video-Emphasis Encoding Apparatus]
In a known video-emphasis encoding apparatus, an input video signal is subjected to spatial high-pass filtering for extracting high-frequency (HF) components. The extracted HF components are added to the input video signal to produce a HF-component-emphasized signal in accordance with emphasis data indicating an emphasis processing. The emphasis processing is disclosed, for example, in Japanese Un-examined Patent Publication No. 7-162862. The emphasized signal is subjected to a known predictive-encoding processing to produce a variable-length bitstream. The bitstream is multiplexed with the emphasis data. The multiplexed bitstream is then output for storage or transfer.
[Video-Deemphasis Decoding Apparatus]
In a known video-deemphasis decoding apparatus compatible with the video-emphasis encoding apparatus described above, the emphasis data is separated from the multiplexed bitstream. The remaining bitstream is subjected to a known decoding processing to reproduce a predictive error that has been produced by the predictive encoding. The reproduced predictive error is subjected to spatial high-pass filtering for extracting high-frequency (HF) components of the original video signal. The HF components are subtracted from the predictive error in accordance with the emphasis data, thus reproduce the original video signal having the frequency characteristics identical to that before the emphasis processing applied.
Such known video-emphasis encoding apparatus, however, has the following drawbacks:
The emphasis processing is applied to video signals with no emphasis-level control. The emphasis data transferred to the video-deemphasis decoding apparatus indicates only whether or not the emphasis processing has been applied to video signals.
An overrange condition thus often occurs to input video signals when the video signals have already been applied the enhancement processing.
Narrow quantization step width for quantization of the input video signals in video-emphasis encoding often causes increase in codes generated against noise elimination.
Moreover, videos signals reproduced by the video-deemphasis decoding apparatus are sometimes unsuitable for the enhancement processing at a display apparatus for displaying the reproduced video signals.